For producing a semiconductor device, a technique called “photolithography” of transferring a circuit pattern onto a wafer by use of a photosensitive resin is used as a technology of forming a fine pattern. Especially by the cutting-edge technology of photolithography, an ArF excimer laser having a wavelength of 193 nm is used, and a copolymer containing an alicyclic monomer having a crosslinked structure in a repeating unit is used as a resist in consideration of light transmittance and anti-etching characteristic (see, for example, Patent Documents 1 through 3). Among alicyclic monomers, an adamantane derivative has a stable and firm structure, is highly symmetrical structurally, and can provide various properties depending on the functional group introduced, and therefore, is widely used.
Recently, the densities of semiconductor devices are becoming increasingly higher. In order to form higher density patterns, a copolymer usable for a resist which can reduce the amount of impurities to the minimum possible level and thus can improve the production yield of the semiconductor devices are desired. Impurities contained in a copolymer include polymerizable impurities which are mixed as a contaminant during production thereof or derived from a monomer as a material of the copolymer. Among such impurities, monomer- or esterification agent-derived homopolymers or oligomers reduce sensitivity or resolution during the photolithography process, or reduce the yield rate, for example, increase the line edge roughness (LER). Such polymerizable impurities also reduce the solubility of the copolymer in a photoresist solvent. This causes defects of a lithographic pattern, such as insufficient spin-coating, increase of developing defects after exposure/development or the like. Such defects of the lithographic pattern directly influence the production yield rate of the semiconductor devices. For these reasons, it is considered not to preferable to use a copolymer containing polymerizable impurities as a material for producing the photoresist. Various proposals have been made in order to reduce the amount of the polymerizable impurities (see, for example, Patent Document 4). However, the polymerizable impurities brought into the copolymer from the monomer used cannot be completely removed on or after the stage of polymer production. Therefore, it is required to reinforce the quality control on the stage of monomer production. In addition, gel permeation chromatography (GPC) analysis, which is used for analysis of the polymerizable impurities, cannot accurately measure the concentration of, and therefore cannot properly control, the polymerizable impurities containing an adamantyl group because such polymerizable impurities have a low solubility in a solvent used in the gel permeation chromatography analysis. In such circumstances, it has been desired to develop a monomer having a further improved quality and a high purity copolymer suitable for production of the next-generation semiconductor devices.